The authors present the method for determining the coefficient of roughness using the modified Manning’s and Erung’s equation in the article. The density of plants in the area is one of the parameters influencing the roughness coefficient and the water table decline as well. It’s necessary to use the roughness factor taken from the Ven Te Chow’s tables in the hydraulic calculations the effect of plant communities on the flow. The selection of the above coefficient could be ambiguous. This may be the cause of overestimation or underestimation of the flow through the analyzed area. Presented mathematical equations describe the geometric structure of plants community in a flood area and could be an alternative to the classical methods for roughness coefficient estimation.

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